The present invention relates to parking braking systems for vehicles having Electronic Braking Systems (EBS).
With the advent of EBS (see for example EP Patent 0205277), there has been a step change in the way that braking is controlled and distributed on heavy commercial vehicles. However, except in some specific areas such as continuous pad wear sensing, the brakes themselves have yet to take advantage of this change.
The conventional approach to parking brakes in vehicles fitted with EBS uses spring brake actuators 12 as shown diagrammatically in FIG. 1 of the accompanying drawings. In this system, a hand-operated valve 10 is used, via a relay valve 11, to allow a parking brake to be applied. The hand-operated valve 10 operates on an inverse air principle in that the hand-operated valve 10 is arranged to release air pressure to allow the spring force of respective spring brake actuators 12 at each wheel to be applied. A suitable parking brake reservoir 14 is required to store the pressurized air for use within the system. Where the system is used with a vehicle having a trailer, a separate relay valve (not shown) is required to allow selective operation of trailer brakes. When the driver operates the hand-operated valve 10, an inverse pneumatic signal is produced, i.e. the pressure output from the hand-operated valve 10 falls with increasing demand. This causes the spring brake actuators 12 to be applied since, in the normal running mode (no braking) the spring brakes are held off by compressed air.
As evident from FIG. 1, the layout and construction of the conventional parking brake system requires the use of bulky spring actuators 12, a parking reservoir 14 and associated pipework. All of these components require fitting and service which all adds to labor and material costs for OEMs and end users.
The present invention seeks to make better use of the facilities afforded with EBS to enable an improved parking brake system.
U.S. Pat. No. 5,127,495 discloses an electrically powered parking braking system that includes a hydraulic drum brake having two brake shoes which are movable outwardly relative to a friction surface of a drum, and an activation device for generating an activation-force to move the shoes outwardly to a first position into contact with the drum sufficiently to prevent the drum from rotating. This system also includes a solenoid activated linkage mechanism for maintaining the shoes substantially in the first position. Operation of the hydraulic drum brake is achieved via a proportional controller using electrical signals generated in response to brake master cylinder pressure or brake pedal effort.